Session Initiation Protocol (SIP) is rapidly becoming the de facto signaling protocol for establishing, modifying and terminating multimedia sessions between users in a communication network. SIP is described in J. Rosenberg et al., “SIP: Session Initiation Protocol,” Internet Engineering Task Force (IETF) RFC 3261, June 2002, which is incorporated by reference herein. SIP has also been adopted for the IP Multimedia Subsystem (IMS), which is the next-generation core network architecture for mobile and fixed services defined by the 3rd Generation Partnership Project (3GPP).
A network element that processes and forwards SIP messages is called a proxy server in SIP terminology, and a Call Session Control Function (CSCF) in IMS terminology. 3GPP defines three types of CSCF elements: Proxy CSCF (P-CSCF) which is the interface to the user, Interrogating CSCF (I-CSCF) which provides an interface to other servers in different administration domains, and Serving CSCF (S-CSCF) which handles registration, enforces policy and provides an interface to application servers. Such network elements are referred herein as SIP/IMS servers, and a signaling network comprising these and other network elements is referred to as a SIP-based network.
In order to achieve high levels of performance in a SIP-based network, it is important to distribute the traffic load evenly over the network elements. Unfortunately, conventional load balancing techniques are often not well suited for use in the SIP context, and may fail to provide the desired performance levels.
A related problem in SIP-based networks involves overload control. Like other network elements, a SIP/IMS server can become overloaded when traffic demand exceeds its available resources, for example, its available processing resources. Even with over-provisioning, overload may still occur for various reasons including temporary traffic surges due to “flash crowd” effect, node or link failures, poor routing, traffic diversion due to maintenance and denial-of-service attacks, etc.
A variety of techniques have been developed for addressing overload control in communication networks. These include, for example, overload control based on M/M/1 queuing systems, and overload control techniques developed for use in the Signaling System 7 (SS7) context.
Unfortunately, these and other conventional overload control techniques fail to address the quantitative impact of overload on SIP performance and fail to provide specific approaches for handling overload in SIP-based networks, which are often more complex in terms of messaging services and signaling topologies.
It is therefore apparent that a need exists for improved load balancing and overload control techniques, particularly in SIP-based networks.